Ledecky, the Olympic, world and Pan Pacs champion, has never lost an international 400m and didn't go under on Saturday night at the Tatsumi International Swimming Centre, taking out the race in 3:58.50s.

But it was the rise and rise of Australia's Titmus, still just a 17-year-old schoolgirl, that set the scene for an epic showdown in the nearby Olympic pool in two years.

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Beginning of a fine rivalry: Katie Ledecky hugs Ariarne Titmus on the podium.Credit:AAP

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Titmus finished second in 3:59.66s, becoming just the third woman to break the four-minute mark in the race. The other is Italian Federica Pelligrini, who was only able to achieve the feat in one of the now-banned supersuits.

Ledecky flirted with her own world record at stages of the race but couldn't shake off Titmus, who was pressing right to the end and finished a bodylength-and-a-half back.

At the Olympics, she wants that margin to be in her favour and after Saturday night, it's no longer an unattainable dream.

“That’s the goal, to be up there with her and hopefully she’ll enjoy having someone to race," Titmus said after the race.

“She hasn’t had anyone to race for a long time, so I’m getting closer. On the sixth lap I pulled her back in a bit but she has a bit more speed than me and she got away a bit on the seventh 50m and not the last 50m. I did the best I could."

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As for Ledecky, her message was simple: bring it on. The 21-year-old said she welcomed a genuine challenger in an event she has ruled without mercy and it would only push her to lower her own extraordinary times.

"It's exciting for me to see that. I was the only one under four minutes in a textile suit for a few years now," Ledecky said.

"I think it's exciting for me to see how I've put the standard out there and I know that a lot of girls are chasing that. It's good to see someone get under it [four minutes] and it's going to push me to go even faster and set the benchmark a little higher."

The pair won't meet again until the FINA World Championships next year, by which time Titmus will be almost 19. Ledecky will be determined to fend off the young Tasmanian, who trains in Brisbane under Dean Boxall, a coach whose star rises alongside that of his pupil.

Ledecky's world record of 3:56.46s was set in Rio, meaning Titmus still has an ocean to cross if she's to beat the American great at her best. But with seconds dropping almost every time she gets in the pool, the stage is most certainly set for a bold new rivalry in the pool.